Bipolar plate and fuel cell comprising such a bipolar plate

ABSTRACT

The invention relates to a bipolar plate, for fuel cells, characterised in comprising a layer of a hydrophobing material which is soluble in a solvent, on the surfaces thereof. Water forms small droplets on the surfaces of the bipolar plate due to said layer, which are loosely held on the surface of the bipolar plate and which can be reliably removed from the fuel cell even with low flow speeds for the operating gases. The thickness of the layer and thus the hydrophobicity thereof and the electrical contact resistance between the bipolar plate and a contacting electrode may be adjusted in a simple manner, by varying the concentration of the hydrophobing material in the solvent.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Stage of International ApplicationNo. PCT/EP2004/003922, filed Apr. 14, 2004 and claims the benefitthereof. The International Application claims the benefits of Germanapplication No. 10323880.8, filed May 26, 2003, both applications areincorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to a bipolar plate for fuel cells and furthermoreto a fuel cell comprising such a bipolar plate.

SUMMARY OF THE INVENTION

Fuel cells are becoming increasingly important in future-orientedconcepts for generating energy. In particular low-temperature fuel cellsbased on Polymer Electrolyte Membrane (PEM) technology are beingdiscussed as environmentally friendly and efficient energy convertersfor portable, mobile and stationary applications and are already findinginitial commercial applications. At temperatures from just abovefreezing point to around 90° C. low-temperature fuel cells converthydrogen and oxygen into electrical direct current, producing water asthe only byproduct.

The major element of an individual PEM cell is a membrane electrodeunit. This consists of two electrodes (an anode and a cathode) and of anelectrolytic membrane situated between the two electrodes. Between theelectrodes and the electrolytic membrane there is a catalyzer layer inwhich the main physical and electrochemical process, such as adsorptionof hydrogen and oxygen at the catalyzer, emission and acceptance ofelectrons and the formation of water occurs on the cathode side throughcombination of protons diffused through the membrane and (reduced)oxygen.

Water or moisture is present in the fuel cell for a number of reasons:On the one hand water is produced as a product on the cathode side, inaddition it can diffuse from the cathode side to the anode side as aresult of a concentration gradient. A further important reason for thepresence of water is that the media hydrogen and oxygen must behumidified up to 100% pure humidity, since only in this way is a goodproton conductivity and a corresponding long-term stability of theproton-conducting membrane used produced.

The gases involved in the reaction are conducted by the electrodes;Simultaneously however the gaseous and especially liquid water which isformed in the operation of the fuel cell or is introduced into the fuelcell by the humidifying process of the gases, must be transported asquickly as possibly by the electrodes away from the electrochemicallyactive catalyzer layer and out of the cell.

In a fuel cell stack the electrodes are located on the side facing awayfrom the electrolytic membrane or catalyzer layer in contact with whatis known as a bipolar plate or cooling unit in each case. The task ofthis component is to separate the individual fuel cells (on the mediaside) to look after flows of current in the cell stack and to remove thereaction heat. To guarantee and effective current flow, the bipolarplates consist of an electrically-conductive material which must exhibita low contact resistance to the electrodes.

If there are low flow velocities of the operating gases hydrogen andoxygen or air in the fuel cell, as is especially the case for fuel cellsoperated with oxygen, and in this case again especially under part loadoperation, the problem arises of water not being able to be reliablytransported out of the fuel cell, since it remains adhering to thesurface of the bipolar plate and cannot be expelled from the fuel cell.This can adversely affect the current flow between the bipolar plate andthe electrodes and thereby the power of the fuel cell.

It is therefore the object of the present invention to improve a bipolarplate such that water can also be securely removed from the fuel cell atonly low flow velocities of the operating gases, with the contactresistance between the bipolar plate and an adjacent electrode notdeteriorating or only deteriorating slightly.

This object is achieved by the claims. Advantageous embodiments are theobject of the dependent claims in each case.

Through the inventive layer of hydrophobing material provided on thesurface of the bipolar plate, water which comes into contact with thebipolar plate forms small droplets which only adhere loosely to thebipolar plate and can thus at only low flow velocity of a gas flowingpast be taken up by the gas flow and removed from the fuel cell.

Because of its solubility in a solvent the hydrophobing material can beapplied in a dissolved state using a simple and normal applicationmethod, such as spraying, wiping, brushing, dipping, pressure etc. tothe bipolar plate. After the vaporization of the solvent and ifnecessary immobilization of the remaining materials by a temperaturestep at increased temperature the desired layer from the hydrophobingmaterial remains on the surface of bipolar plate.

With the aid of the concentration of the hydrophobing material in thesolvent the thickness of the hydrophobing layer and thereby thehydrophobicity and the electrical contact resistance between the bipolarplate and an adjacent electrode can be adjusted in a simple way. A smallconcentration of the hydrophobing material in the solvent leads to alayer with low hydrophobicity and almost constant electrical contactresistance. A higher concentration of the hydrophobing material producesa correspondingly thicker layer with correspondingly higherhydrophobicity, but with a deterioration of the electrical contactresistance.

A desired lower electrical contact resistance is in this case solelyobtained by the concentration of the hydrophobing material in thesolvent and the layer thickness thus produced; Additional metalliccomponents in the layer for reducing the contact resistance are notnecessary nor is there provision for them.

The hydrophobing material preferably consists entirely or partly of anamorphous fluoropolymer. Amorphous fluoropolymers, e.g. amorphousmodifications of Teflon, can be applied already dissolved in suitablesolvents and thinned before use to an optimum concentration.

In a further advantageous embodiment of the invention there is provisionfor the hydrophobing material to consist entirely or partly of apolysiloxane compound or from alkylsilanes, especiallyalkyl-aryl-silanes or halogen alkyl-aryl-silanes. Such compounds alsostand out by virtue of their good solubility in solvents and goodadhesion to metal surfaces.

To simultaneously achieve just a slight effect on the contact resistancebetween the electrodes and the bipolar plate as well as a good removalof water from the fuel cell, an advantageous embodiment of the inventionprovides for the thickness of the layer of hydrophobing material to beset to an optimum between a low electrical contact resistance to anadjacent electrode and a high permanent hydrophobicity.

A low contact resistance and a simultaneous good hydrophobicity of thelayer can be achieved by the hydrophilic layer having a thickness in therange of 0.1 nm to 50 nm, especially in the range 0.5 nm to 5 nm.

A reduction of the transfer resistance between the bipolar plate and theelectrode is still possible by providing a highly-conducting contactlayer, especially made of gold, between the bipolar plate and the layerof hydrophobing material.

The invention as well as further advantageous embodiments of theinvention in accordance with features of the subclaims is explained inmore detail below with reference to an exemplary embodiment in theFIGURE.

BRIEF DESCRIPTION OF THE DRAWING

The sole FIGURE shows an exemplary embodiment of the invention.

DETAILED DESCRIPTION OF INVENTION

The FIGURE shows a schematic and simplified presentation of a sectionthrough a stack 10 of low-temperature fuel cells 1. The stack 10comprises a number of alternating bipolar plates 5 andmembrane-electrolytic electrode units 3 stacked on top of each other.The membrane-electrolytic electrode units 3 each consist of an anode 7,a Polymer Electrolytic Membrane 9 and a cathode 11. A catalyzer layer 8is located between the anode 7 or the cathode 11 and the membrane 9. Theanodes 7 and cathodes 9 preferably consist of carbon paper. A bipolarplate 5 can also be embodied as a cooling card and to this end canfeature a cavity through which cooling water flows.

The bipolar plates 5 advantageously consist of a metal alloy, especiallya nickel-based alloy or a copper alloy and thereby exhibit a goodelectrical conductivity.

The bipolar plates 5 feature bumps 22 on their surface. These formcathode gas spaces 23 between a cathode 11 and an adjacent bipolar plate5 which are connected to each other in each case (not shown) and allow athroughflow and a propagation of oxygen or air over the entire surfaceof the cathode. In a corresponding way anode gas spaces 25 are formedbetween an anode 7 and an adjacent bipolar plate 5, with the anode gasspaces 25 each being connected to each other (not shown) and allowingpropagation of the hydrogen over the entire surface of the anode. At thepoints at which the bipolar plate 5 is in direct contact with an anode 7or cathode 11, the electrical current flows between the two components.

During the operation of a fuel cell 1 wetted hydrogen flows from a feedchannel 27 into the anode gas space 25 and is removed again via adischarge channel 28 from the fuel cell 1. In addition the oxygen or airwetted with water flows via a feed channel 29 into the cathode gas space23 and is removed via a discharge channel 30 from the fuel cell 1.

On the electrode side the bipolar plates 5 feature on their surface alayer 4 of a hydrophobing materiel soluble in a solvent. Preferably thislayer 4 consists entirely or partly of a amorphous fluoropolymer,especially an amorphous modification of Teflon (for example an amorphouscopolymer made of 65-99 Mol Perfluoro-2,2-Dimethyl-1,3-Dioxol with acomplementary quantity of Tetrafluor ethylene available from DuPontFluoroproducts under the product name Teflon®AF). Alternatively thelayer 4 consists of a polysiloxane compound or alkylsilanes, especiallyalkyl-aryl-silanes or halogen-alkyl-aryl silanes.

For improving the contact resistance between the bipolar plate 5 and theadjacent anode 7 or cathode 11 there is an additional highly-conductivecontact layer 6 made of a noble metal between the hydrophobing layer 4and the bipolar plate 5.

Through the water arising in the operation of the fuel cell or waterintroduced through the wetting process of the gases into the fuel cell 1the bipolar plate 5 comes into contact with water both on the cathodeside and also on the anode side. Because of the hydrophobicity of thelayer 4 on the surface of the bipolar plate 5 the water forms droplets12, 13 which are small and easily removable from the surface. Thecathode-side droplets 12 can thus be transported at just a slight flowvelocity of the oxygen or the air in the cathode gas space 23 in theflow direction 32 from this space and removed via the discharge channel30 from the fuel cell. In a corresponding way the anode-side droplets 13can also be transported at only a low flow velocity of the hydrogen inthe anode gas space 25 in the direction of flow 31 out of this space andremoved via the discharge channel 28 from the fuel cell.

The thickness of the hydrophobing layer 4 is in this case adjusted to anoptimum between a low electrical contact resistance of the cathode 11 orthe anode 7 to the bipolar plate 5 and a high hydrophobicity of thelayer 4 and amounts to 0.5 nm to 5 nm. The layer thickness is adjustedin this case through corresponding thinning of the hydrophobing materialin a suitable solvent.

A bipolar plate which makes reliable removal of water from the fuel celland simultaneously a small electrical contact resistance to an adjoiningelectrode can be achieved by the following steps:

2.5 ml of a 6 (% by weight) solution of Teflon®AF 1601-56 from themanufacturer DuPont Fluoroproducts is added into a manufacturer-suppliedsolvent FC75 in a 500 ml measuring cylinder with a perhalogenatedsolvent FC77 (manufacturer:3M) to 500 ml and 12 stirred for 12 hours atroom temperature using a magnetic stirrer. The solution is then readyfor use and is applied to a cleaned and dried bipolar plate made of anickel-based alloy, provided with a contact coating made of gold, usinga sponge by even distribution. Subsequently the layer is dried for 30minutes at 100° C. in the recirculating oven. A layer remains on thebipolar plate with of a thickness of appr. 0.6 nm.

Bipolar plates treated in this way exhibit by comparison with untreatedplates a markedly different behavior in relation to water, since waterapplied to it forms droplets which only adhere slightly to the surfaceof the bipolar plate and can be blown away with only a slight gas flow.

With bipolar plates modified in this way short stacks are constructedwith suitable membrane electrolytic electrode units and operated inhydrogen/oxygen mode. Characteristic fuel cell data such as for examplea flow density of 1 A/cm2 at a voltage of 0.7 V for an individual cellfor operating times of more than 1500 hours. The electrical data therebydiffers from that of corresponding fuel cells which are operated withconventional bipolar plates. This shows that because of the smallthickness of the hydrophobing layer the contact resistance between thebipolar plate and an adjoining electrode are not or are only veryslightly influenced. In this case no accumulation of water can beestablished on the surface of the bipolar plate in this case. This showsthat this water can be reliably discharged from the fuel cell.

1-9. (canceled)
 10. A bipolar plate for fuel cells, wherein the bipolarplate is provided on its surface with a layer of a hydrophobing materialsoluble in a solvent.
 11. The bipolar plate in accordance with claim 10,wherein the hydrophobing material comprises entirely or partly anamorphous fluoropolymer.
 12. The bipolar plate in accordance with claim10, wherein the hydrophobing material comprises entirely or partly apolysiloxane compound or alkylsilanes.
 13. The bipolar plate inaccordance with claim 12, wherein the alkylsilanes arealkyl-aryl-silanes or halogen-alkyl-aryl-silanes.
 14. The bipolar platein accordance with claim 10, wherein a thickness of the layer isadjusted to an optimum between a low electrical contact resistance to anadjoining electrode and a high hydrophobicity.
 15. The bipolar plate inaccordance with claim 11, wherein a thickness of the layer is adjustedto an optimum between a low electrical contact resistance to anadjoining electrode and a high hydrophobicity.
 16. The bipolar plate inaccordance with claim 12, wherein a thickness of the layer is adjustedto an optimum between a low electrical contact resistance to anadjoining electrode and a high hydrophobicity.
 17. The bipolar plate inaccordance with claim 10, wherein a thickness of the layer ranges from0.1 nm to 50 nm.
 18. The bipolar plate in accordance with claim 17,wherein the thickness of the layer ranges from 0.5 nm to 5 nm.
 19. Thebipolar plate in accordance with claim 11, wherein a thickness of thelayer ranges from 0.1 nm to 50 nm.
 20. The bipolar plate in accordancewith claim 12, wherein a thickness of the layer ranges from 0.1 nm to 50nm.
 21. The bipolar plate in accordance with claim 14, wherein athickness of the layer ranges from 0.1 nm to 50 nm.
 22. The bipolarplate in accordance with claim 10, wherein the bipolar plate comprises ametallic alloy.
 23. The bipolar plate in accordance with claim 22,wherein the metallic alloy is a nickel-based alloy.
 24. The bipolarplate in accordance with claim 11, wherein the bipolar plate comprises ametallic alloy.
 25. The bipolar plate in accordance with claim 10,further comprising a highly-conductive contact layer between the bipolarplate and the layer made of the hydrophobing material, wherein thehighly-conductive contact layer is made of a noble metal.
 26. Thebipolar plate in accordance with claim 25, wherein the noble metal isgold.
 27. A fuel cell, comprising: a membrane-electrode unit; and abipolar plate electrically contacting the membrane-electrode unit on theelectrode side, wherein the bipolar plate is in accordance with claim10.